The invention is based on a priority application (DE 100 29 733.1) which is hereby incorporated by reference. The invention relates to an antenna arrangement (flat antenna arrangement, plate antenna arrangement, patch antenna arrangement) with an earth plate and a radiator that is disposed at a distance essentially parallel to the earth plate and is conductively connected to the latter by one of its end regions, wherein, at a first resonant frequency of the antenna arrangement, a voltage minimum is present at the connection of the radiator to the earth plate and in the region of the other end (unsupported end) of the radiator a first voltage maximum is present.
Built-in antennas for mobile-radio telephones are known that are based on the principle of the patch antenna. The external dimensions of such an antenna module are minimized in existing applications, for example, by using a folded structure (for example, C-patch). In addition to the singly resonant design (a single operating frequency band), further structures are also known that make possible operation in two defined frequency bands (such as, for example, in the two mobile-radio bands of the GSM900 and of the GSM1800 standards). In this case, either two separate radiators are used or the result is achieved by suitable measures that only a certain radiator part is used at the higher operating frequency. These procedures have the disadvantage that the entire available antenna volume is not used, in particular at the higher frequency. This results in a low bandwidth of the antenna.
The object of the invention is to design an arrangement of the type mentioned at the outset in such a way that it is suitable for two frequency ranges and permits a wide-band structure.
This object is achieved in that there is disposed between the radiator and the earth plate near the unsupported end of the radiator a settable switch element that is designed in such a way that it is capable of making a connection having a low impedance and in that the point at which the switch element is connected to the radiator is disposed in such a way that, with the switch element controlled to conducting, the radiator has a desired second resonant frequency that is higher than the first resonant frequency. An advantage of the invention is that the entire or almost the entire radiator radiates in the two frequency ranges. As a result, a relatively large bandwidth is possible even at the higher frequency because a large radiator surface is available. At the lower frequency, too, there is an advantage because here, again, the entire area available for the antenna in total can be used as radiator. A single point of the radiator can be used for the feed.
One embodiment of the invention provides that the switch element is disposed in such a way that the second resonant frequency corresponds to about twice the first resonant frequency. This ratio of the resonance frequencies is well suited to implementing a mobile telephone for two-band operation, for instance in the GSM900/GSM1800 or GSM900/GSM1900 range.
In one embodiment of the invention, the radiator essentially has the configuration of a C, with the inclusion of an approximately C-shaped form having a non-round, angular shape. This has proved beneficial.
Proceeding from the antenna arrangement described at the outset, the object of designing such an arrangement in such a way that it is suitable for two frequency ranges and permits a broad-band structure is achieved in that the radiator has essentially the shape of a meander or of a plurality of consecutive conductor sections disposed in a zigzag shape, in that, at a further, higher resonant frequency, a voltage minimum or a second voltage maximum, respectively, is present at the said ends of the radiator, and in that such a point of the radiator is capacitively coupled to the earth plate so that the further resonant frequency is reduced with respect to three times the value of the first resonant frequency. This relates to configurations of the radiator that may in the individual case have advantages over a C-configuration. One advantage is that the entire or almost the entire radiator radiates in the two frequency ranges. As a result, a relatively large bandwidth is possible also at the higher frequency because a large radiator surface is available. There is also an advantage at the lower frequency because here, again, the entire area available in total for the antenna can be used as radiator. A single point of the radiation can be used for the feed.
In one embodiment of the invention the radiator has essentially an S-like shape in which three sections extend approximately in the transverse direction of a rectangular surface enclosing the radiator, wherein two sections in each case are connected by a total of two connecting sections. This is a special configuration.
In one embodiment of the invention the capacitance value and the said point of the radiator are chosen in such a way that the first resonant frequency is less strongly reduced than the second resonant frequency. It is advantageous that the antenna can be kept small in its dimensions.
In one embodiment of the invention the capacitance value and the connection of the capacitive coupling are chosen in such a way that the second resonant frequency corresponds, as a rough approximation, to twice the first resonant frequency. The suitability for operation in the 900/1800 MHz or 900/1900 MHz bands is advantageous.
In one embodiment of the invention the said other point of the radiator with which the capacitive coupling takes place is situated in the vicinity of the first voltage maximum on the radiator at the second resonant frequency. A particularly strong reduction in the second resonant frequency with a small reduction in the first resonant frequency is advantageous.
In one embodiment of the invention the said other point is situated approximately at ⅓ of the developed length of the radiator measured from the connection to the earth plate. This is a beneficial dimensioning in many cases.